Symmetry reduction in group 4mm photonic crystals

The size of absolute band gaps in two-dimensional photonic crystals is often limited by band degeneracies at the lattice symmetry points. By reducing the lattice symmetry, these degeneracies can be lifted to increase the size of existing photonic band gaps, or to create new gaps where none existed for the more symmetric structure. Specifically, symmetry reduction by the addition of different diameter rods into the unit cell of two-dimensional square lattices (Laue group 4mm) is explored. This approach is especially useful in opening absolute band gaps in structures of dielectric rods in air, which are more easily microfabricated than a crystal of air columns in a dielectric background. Symmetry reduction offers a rational approach for exploring and designing new photonic crystal structures.